The ''Yellow Trains'' or to be more accurate, the various measurement and test trains operated by Network Rail are a regular sight to commuters and passengers alike on all parts of the railway system. Operated by Network Rail on behalf of any number of the various companies charged with maintaining the network, their task is to check, assess and monitor the track, bridges and tunnels on both passenger and freight only lines. In the past these checks were done principally by staff on foot, and were naturally limited to the distance it was possible to cover in a set period, or by the visual observation of bridges and tunnels often using an old coach converted for inspection purposes. Today''s high-speed railway demanded something different and as such the various test trains have evolved filled to the roof with the most sophisticated equipment to be found anywhere. Speeding along the lines at up to 125 mph, they will check the alignment of the track and distance to structures, as well as locating defects for general care and maintenance for later attention or, if the defect is serious enough, immediate attention, in which case all following traffic is stopped until a detailed check and if necessary rectification is made. Author Andrew Royle spent many years engaged as a computer technician on a variety of the different test trains and in the course of his travels covered much of the network. His story is both technical and personal, from the purpose of the equipment and how it operates to the trials and tribulations of using it.